The present invention relates to a device for transmitting the movement to fans for cooling the cooling fluid in vehicles, arranged coaxially with the driving shaft thereof.
It is known in the technical sector relating to the cooling of cooling fluids contained in motor-vehicle radiators that there exists the need to force air onto the radiator in order to obtain more rapid dissipation of heat from the fluid to the exterior, said forced air flow being obtained by causing rotation of a fan which is normally mounted on the water pump shaft or on the driving shaft or on a driven and fixed shaft carrying a pulley which receives movement from a belt actuated by the driving shaft.
It is also known that said fan must be made to rotate only upon reaching a certain predefined temperature of the water detected by means of a thermostat which activates an electromagnetic clutch, closing of which causes the fan to start rotating.
More particularly it is required that a motor vehicle fan must be able to rotate:                at a lower speed than that of the transmission shaft for cooling in low external temperature conditions;        at a speed equal to or even greater than that of the transmission shaft in the case of higher external temperatures or use in severe conditions which cause overheating of the engine;        at zero speed, namely with the fan which does not rotate at all and remains in an idle condition with respect to the transmission shaft, in the case of particularly low temperatures at which further cooling is of no use or even damaging.        
In an attempt to achieve these performance features, coupling systems of the mixed type with electromagnetically operated friction clutches and drive couplings based on the use of parasitic currents generated by rotation of a conducting element in the vicinity of permanent magnets have been developed.
DE-32 03 143 describes, for example, an arrangement in which the driving shaft is connected to the rotor of an electromagnetic clutch, which is engaged by an armature connected to the fan for direct driving, whereas slow speed operation makes use of the engagement between a conducting disk, rotating with the transmission shaft, and the permanent magnets integral with the fan, said engagement causing transmission of movement at a low speed as a result of relative slipping between the two parts. With this solution, however, it is not possible to obtain the idle condition of the fan.
In addition to this, the known devices do not envisage the possibility of maintaining an albeit slow rotation of the fan in the case of breakage and/or complete interruption of the power supply to the coils of the clutches as occurs for example in the event of a total electrical failure.
Said devices are applied to cooling fans, the axis of rotation of which is arranged in a position slightly higher than the axis of the driving shaft in order to limit the radial dimensions of the fan and obtain rotational speeds thereof greater than those of the driving shaft.
The evolution of vehicles, in particular large-size vehicles such as lorries and the like, however, is progressing in such a way that it is preferred to design the driving cabs so that they are arranged as close as possible to the ground in order to reduce the friction and improve the air penetration coefficient of the vehicle; this arrangement of the cab, however, is made difficult by the said raised position of the fan which therefore must be displaced into a position substantially coaxial with the driving shaft.